In communications networks interconnecting a plurality of exchanges (private or public), calls are distributed amongst channels. Thus, in more or less general terms, so-called “B” channels are known which are general-purpose channels for conveying messages between various parties. The parties may be people in which case the messages can be voice messages. The parties can also be machines, in which case the messages will be digitized data messages. In practice, analog transmission for messages between people is being abandoned in favor of digital transmission, since transmission quality is better.
Besides such general-purpose channels, there are so-called “D” channels which serve to carry signaling data. In operation exchanges, or switches, need to send each other messages concerning their availability and their mode of operation. For example, if a telephone exchange is connected to a base station of a time division multiple access (TDMA) type mobile telephone network, it is necessary to transmit synchronization, and to transmit the time to the various mobile stations which seek to come into contact with the base station. In another example, in the X25 protocol, before a message can be sent, it is necessary to deliver information to the circuits that are in the traffic concerning the time position and the encapsulation data that is to be placed around the message.
To simplify explanation, it is recalled that in a digitized communications system in France there exist so-called “T2” services which thus comprise for the user thirty “B” channels for conveying speech, and one “D” channel for conveying signaling messages.
The problem of managing signaling messages is essentially associated with the protocol which organizes a link between two telephone exchanges. For any one such link, the protocol is known by the two exchanges involved, and signaling messages can normally be conveyed between them. If a signaling message needs to be forwarded to another exchange using a link that has the same protocol for transmitting signaling messages, then the signaling message can be forwarded as such. It will be correctly interpreted by the end exchange.
However, a problem arises within a network that is 20 not uniform, in that it contains a variety of communications channels. Under such circumstances, either the signaling message cannot be conveyed or, in order to enable it to be conveyed, an exchange in the network at which links using different protocols terminate must include as many transcoders for forwarding signaling messages as there are pairs of different transmission protocols for such signaling messages. Given the present wide variety of signaling channels, and given the wide variety of protocols that can be used within these channels, such signaling message transcoding is not undertaken. The whole advantage of signaling channels is lost once the network is not uniform.